1. Field of the Invention
This invention relates to electrical circuit overcurrent protection.
2. Introduction to the Invention
Positive temperature coefficient (PTC) circuit protection devices are well known. The device is placed in series with a load, and under normal operating conditions is in a low temperature, low resistance state. However, if the current through the PTC device increases excessively, and/or the ambient temperature around the PTC device increases excessively, and/or the normal operating current is maintained for more than the normal operating time, then the PTC device will be xe2x80x9ctripped,xe2x80x9d i.e. converted to a high temperature, high resistance state such that the current is reduced substantially. Generally, the PTC device will remain in the tipped state, even if the current and/or temperature return to their normal levels, until the PTC device has been disconnected from the power source and allowed to cool. Particularly useful PTC devices contain a PTC element which is composed of a PTC conductive polymer, i.e. a composition which comprises (1) an organic polymer, and (2) dispersed, or otherwise distributed, in the polymer, a particulate conductive filler, preferably carbon black. PTC conductive polymers and devices containing them are described, for example in U.S. Pat. Nos. 4,237,441, 4,238,812, 4,315,237, 4,317,027, 4,426,633, 4,545,926, 4,689,475, 4,724,417, 4,774,024, 4,780,598, 4,800,253, 4,845,838, 4,857,880, 4,859,836, 4,907,340, 4,924,074, 4,935,156, 4,967,176, 5,049,850, 5,089,801 and 5,378,407, the disclosures of which are incorporated herein by reference for all purposes.
In a batch of PTC devices made by the same manufacturing process, uncontrollable variations in the process can cause substantial variation in the conditions which will trip any individual device. The largest steady state current which will not cause any of the devices in the batch to trip is referred to herein as the xe2x80x9cpass currentxe2x80x9d (IPASS) or xe2x80x9chold currentxe2x80x9d, and the smallest steady state current which will cause all of the devices to trip is referred to as the xe2x80x9ctrip currentxe2x80x9d (ITRIP). In general, the difference between IPASS and ITRIP decreases slowly as the ambient temperature increases. Depending on the particular type of device, ITRIP may for example be 1.5 to 2.5 times at 20xc2x0 C. For any individual device, the pass current and the trip current are the same. However, in this specification, reference is made to a PTC device having an IPASS and a different ITRIP, because as a practical matter, the manufacturer of an electrical switch must make use of PTC devices taken from a batch of such devices. Generally, the higher the ambient temperature, the lower the pass current and the trip current. This phenomenon is referred to as xe2x80x9cthermal deratingxe2x80x9d, and the term xe2x80x9cderating curvexe2x80x9d is used to denote a graph of temperature against pass current.
A limitation on the known uses of PTC protection devices is that when a PTC device is placed in series with the load and sized to conduct the normal circuit current, the PTC device can take a relatively long time to convert to its tripped state on an overcurrent which is, e.g., up to a few times the normal circuit current.
The invention provides a new overcurrent protection system which will give a rapid response to even relatively small overcurrents. In the new system, a sensor element and circuit interruption element are placed in series with the load. The sensor element is functionally linked to the circuit interruption element via a control element, so that, when the current in the circuit exceeds a predetermined amount, the sensor element senses the overcurrent and communicates with the control element. The control element causes the circuit interruption element to change from a relatively conductive normal state to a relatively non-conductive fault state (including a completely open state). The invention also provides a new relay assembly which is useful in circuit protection arrangements including circuit protection arrangements of the invention. The new relay assembly, comprises a wiper and an electrical contact. When the wiper is in contact with the relay contact, thereby making a connection, the wiper will open the connection when a current through the connection exceeds a predetermined current amount.
In an example of a preferred embodiment of the invention, the functions of the sensor element and circuit interruption element are combined in a sensor-interrupt element and are provided by a relay having a new bimetal wiper which, itself, is an aspect of the invention. When the relay is energized, the bimetal wiper of the relay is placed in series with the parallel combination of the load and the control element. When an overcurrent passes through such a system, the bimetal wiper disengages from the relay contact thereby breaking the circuit to the load and the control element. The control element causes the sensor-interrupt element to latch open in the fault state.
In an aspect, the invention provides a relay assembly which will open a connection when a current through the connection exceeds a predetermined current amount. The relay assembly comprises a wiper and a relay contact, the wiper being (i) in a first position in which it is in contact with the relay contact, thereby making the connection, or (ii) in a second position in which it is separate from the relay contact, and the wiper moving from the first position to the second position, thereby breaking the connection, when a current through the wiper exceeds the predetermined current amount.
It will be apparent that in the preferred embodiments, this invention permits the use of bimetal switches to be arranged with mechanical switches and other electrical devices to provide reliable protection which protection was not previously available in the art. These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiments of the invention is as illustrated in the various drawing figures.